NEON: 5% faster conversion to RGB565 and RGBA4444
NEON: 5% faster conversion to RGB565 and RGBA4444

We use the magic 'shift and insert' instruction instead of
the multiple shifts and or's.

Change-Id: I48df0320668b502a91792defc0423a9441669d19

PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp